Polyisocyanurate adducts of polyisocyanates are well-known polyurethane intermediates used in the preparation of high performance urethane coatings, paints, and films. These adducts provide improved physical properties when used in such applications, as compared to dysfunctional isocyanates such as toluene diisocyanate. In addition, these adducts provide reduced volatility and an associated reduced toxicity hazard during use, as compared to toluene diisocyanate.
Processes for preparing these adducts are well known. Examples illustrative of these processes can be found in U.S. Pat. Nos. 4,220,728; 4,265,798; 4,324,879; and 4,412,073. Generally, the prior art processes involve adding a catalyst which promotes the isocyanate to isocyanurate (also known as "trimerization") reaction to the precursor isocyanate, optionally in the presence, but usually in the absence, of a solvent, allowing the reaction to proceed to the desired extent and then stopping the reaction with a suitable quenching reagent which destroys the activity of the catalyst. After the residual, unreacted precursor isocyanate is removed, the resulting material, in the case where the precursor isocyanate is a diisocyanate, is a mixture of oligomers composed of 3, 5, 7, etc. precursor diisocyanate molecules joined by 1, 2, 3, etc. isocyanurate rings. Usually, this mixture is simply called "trimer".
In the case where the precursor isocyanate is polyisocyanate, the reaction is stopped well before all the isocyanate groups have been converted to isocyanurate groups because, otherwise, the resulting product would be an unusable polymer having a very high (theoritically infinite) molecular weight and viscosity. However, the cost of equipment and energy to remove residual, unreacted precursor isocyanate dictate that the reaction not be stopped too soon. Generally, the reaction is run to more than 10% conversion but less than 50% conversion. The preferred range is between 25 and 35%. The resulting mixture of oligomers has a viscosity that is low enough to be suitable for some applications. However, for many uses, especially spray-coating and spray-painting applications, the viscosity of the product must be reduced with solvents. The use of solvents is often not desired because of their attendant hazards to workers and the environment. A preferred approach would be embodied in an adduct mixture of inherently lower viscosity, specifically, one containing a higher concentration of monomeric and low molecular weight oligomeric isocyanurate adducts and a lower concentration of the high molecular weight oligomers.
A collateral problem in the trimerization reaction, namely the formation of color species in the product, can arise if the reactants, solvents and equipment are not scrupulously clean, pure and free of contaminants, especially oxygen. Since these conditions can not always be achieved, it would be desirable to have a method to treat the product so as to free the product from the color species. This would provide assurance that the trimer product would not impart an undesirable color to the final coating or film.
Methods for producing high purity polyisocyanates containing an isocyanurate moiety (so-called "isocyanurate polyisocyanates") are known in the art. By way of illustration, U.S. Pat. No. 4,963,675 discloses a process for extracting unreacted diisocyanate monomer and diisocyanate dimer, that is, the product formed from two molecules of diisocyanate monomer joined by an uretidione ring, from a cyclotrimerized diisocyanate by extracting the impure cyclotrimerized diisocyanate with an inert gas in either the liquid or supercritical state. Unfortunately, the elevated temperatures and pressures generally required to carry out the process of the '675 patent entail high equipment and operational costs which make this process less satisfactory a commercial standpoint than might be desired.
In addition, the product mixtures obtained from the process of the '675 patent will still contain any high molecular weight oligomers and any color bodies that may have been present in the starting material. As such, they will tend to have higher viscosity and/or a greater amount of discoloration (e.g., yellow color) than is desired for many applications. Unfortunately, no methods are currently known, to the knowledge of the current inventors, for separating these higher molecular weight oligomers from the desired, low molecular weight, "monomeric" trimer, that is, the product formed from three molecules of diisocyanate monomer joined by one isocyanurate ring. Accordingly, new methodology which provides polyisocyanurate adducts having a higher degree of purity and which are further characterized by reduced viscosity or a combination of reduced discoloration and reduced viscosity, relative to commercially available polyisocyanurate adducts, would be highly desired by the specialty isocyanates manufacturing community. Heretofore, adducts possessing such enhanced properties have not been known based upon the knowledge of the present inventors.